1) Field of the Invention
The present invention relates to an optical device module that homogenizes temperature distribution in optical devices, such as acousto-optic effect selection filters and arrayed waveguide grating filters, which are used in communication systems that employ wavelength-division multiplexing.
2) Description of the Related Art
If wavelength-division multiplexing (WDM) method is used in communication systems, the transmission capacity increases remarkably. Conventionally, in each node that carries out wavelength-division multiplexing, functions of adding and dropping optional wavelengths of light are essential. LiNbO3 waveguide type acousto-optic tunable filters (AOTF) that achieve these functions using acousto-optic effects are attracting attention. The advantage of the AOTF is that multiple wavelengths can be selected simultaneously or wide wavelength band exceeding 100 nanometer (nm) can be selected (see Optorics (1999) No. 5, P155 and The Institute of Electronics, Information and Communication Engineers, OPE 96-123, P79).
In addition, as optical devices for adding and dropping various optical signals, an optical waveguide grating (AWG), which uses an optical add/drop module array, is also included in main devices under examination. The advantage of the optical add/drop module array is that signal wavelength grids having intervals of 0.8 nm to 0.2 nm can be supported in the device.
(1) FIG. 27 illustrates a technique applied in a conventional optical device module (see Japanese Patent Application Laid-Open Publication No. 2001-330811). The AOTF 202 mentioned above includes x-cut LiNbO3 (lithium niobate) substrate 204, waveguide 206 formed by diffusing Ti (titanium) at high temperature by the Ti-diffusion method on LiNbO3 substrate 204, splines 208 formed at the positions on the light incoming side and light outgoing side on LiNbO3 substrate 204, interdigital transducer (IDT) 210 formed by patterning at the position in the light incoming side from the center, polarized beam splitter (PBS) 212 formed on LiNbO3 substrate 204 by the Ti-diffusion method, surface acoustic wave (SAW) guide 211 formed on waveguide 206, and SAW absorber 213.
In the AOTF 202, incoming light λ1 through λn are polarized and separated by PBS 212 and at the same time, polarized and synthesized by PBS 212 again. One beam is outputted to a split light port (not illustrated) as split light λ1 and the other is outputted to a transmitted light port (not illustrated) as transmitted light λ2 through λn. Moreover, incoming light of wavelength equivalent to the frequency of waveguide 206, that is, incoming light λ1 only is polarized and converted by transmitting waveguide 206 and outputted to the split light port.
(2) Japanese Patent Application Laid-Open Publication No. H8-146369 discloses a technique in which an acousto-optic filter includes a light waveguide for propagating single relative rectilinear polarized light, SAW generating means mounted on the optical waveguide for generating the SAW, and an interaction region which distributes a propagation loss of the SAW spatially and converts a specific wavelength component of the single relative rectilinear polarized light propagated in the light waveguide into rectilinear polarized light, which cross at right angles.
(3) Japanese Patent Application Laid-Open Publication No. H11-326855 discloses a method for adjusting filter wavelength characteristics by changing the shape and location of a strain-providing section after manufacturing an element with the strain providing section, for correcting local double refraction index of an optical waveguide.
(4) Japanese Patent Application Laid-Open Publication No. H9-49994 discloses a wavelength filter with an absorber for absorbing an SAW by each reflective electrode to the outside of the optical waveguide by forming the optical waveguide, and excitation electrodes for exciting the SAW on an acousto-optic crystal substrate and disposing reflective electrodes on propagation passage of the SAW.
(5) Japanese Patent Application Laid-Open Publication No. 2002-90563 discloses a waveguide type optical module, in which a heating/cooling element for controlling the temperature of the waveguide type optical element using a soaking plate and heat buffer layer, is installed on the temperature dependent waveguide type optical element and at least part of the soaking plate is brought into contact with the waveguide type element.
(6) On the other hand, Japanese Patent Application Laid-Open Publication No. 2000-249853 discloses an arrayed-waveguide grating that uses an optical add/drop module, as shown in FIG. 28, and that includes a waveguide chip (including, for example, optical substrate such as silicon, quartz, sapphire, etc.) 224 with arrayed waveguide (channel waveguide) 222 provided with optical add/drop functions on the surface, slab waveguide 226, and soaking plate 228 which bonds to the rear surface of waveguide chip 224 and soaks waveguide chip 224, wherein upper plate 230 for optical fiber connection is installed to the surface with arrayed waveguide 222 of waveguide chip 224 formed.
The AOTF 202 of LiNbO3 waveguide type according to (1) generally has heater 234 positioned with soaking plate 232, for example, copper plate, etc. intervened on the rear surface of x-cut LiNbO3 substrate 204 as shown in FIG. 29, and modularized in package (PKG) 236 together with this heater 234.
The heat conductivity in module construction of AOTF 202 can be assumed to be obtained by connecting heat resistance Rln of LiNbO3 substrate 204 as well as ambient (air) heat resistance Rair and heat resistance Rpkg of package 236 in parallel across current source I and external heat source (PKG incoming radiation) Ta as per thermal substrate 204 shown in FIG. 30 when package 236 is formed by material having comparatively high thermal conductivity.
This can be shown by the following mathematical expression:Th=I·((Rpkg·Rair)/(Rpkg+Rln+Rair))+Ta  (1)ΔT =Th−Ta  (2)∴I=ΔT·(1+Rpkg/Rair+Rln/Rair)/Rpkg ≈ΔT·(1/Rpkg)  (3)where Th is the temperature of LiNbO3 substrate 204.
Based on the above thermal conductivity, the temperature distribution of LiNbO3 waveguide type AOTF 202 is investigated. When the thermal conductivity is high, that is, Rpkg is small, soaking can take place, but (3) indicates that consumption power I of heater 234 increases as Rpkg decreases. That is, even if soaking is carried out, the consumption power of heater 234 increases and it becomes unserviceable.
On the other hand, when package 236 is formed by material with low thermal conductivity (see FIG. 31), as is the case of the thermal equivalent circuit shown in FIG. 32A, across current source I and external heat source (PKG incoming radiation) Ta, heat resistance Rln0 of LiNbO3 substrate 204, and ambient heat resistance Rair0 are connected in parallel with heat resistance Rpkg of package 236. Heat resistance Rln1 of LiNbO3 substrate 204 and ambient heat resistance Rair1 in parallel with heat resistance Rpkg of package 236 are further connected in parallel after connecting center heat resistance Rspkg of package 236 across both parallel connections. At the same time, ambient heat resistance Rairout outside package 236 in communication with the ground is connected across heat resistance Rpkg of package 236 and ambient heat resistance Rair1.
This thermal equivalent circuit can be simplified to a circuit as shown in FIG. 32B in which heat resistance Rln0, heat resistance Rair0 of ambient in PKG, heat resistance Rpkg of LiNbO3 substrate 204, and heat resistance Rair1 of ambient in PKG are connected to center heat resistance Rspkg of package 236 in parallel across external heat source (PKG incoming radiation) Ta and heat resistance Rairout of ambient outside package 236.
This can be given by the following mathematical expression:ΔTpkg=Ta·Rc/(Rc+Rairout)  (4)ΔTs=ΔTpkg·Rln/(Rln+Rair)  (5)where, Rln0=Rln1=Rln·½Rair0=Rair1=Rair·½∴ΔTs=(Rc/(Rc+Rairout))·(Rln/(Rln+Rair))·Ta  (6)ΔTs=Ta·(Rpkg/Rairout)·(Rln/Rair)/((Rpkg/Rair+Rln/Rair+1)+(1+Rln/Rair)·Rpkg/Rairout)  (7)
Now if Rln<<Rair or Rpkg<<Rairout,ΔTs=0.
Based on the above thermal conductivity, the temperature distribution of LiNbO3 waveguide type AOTF 202 is investigated. In general, since Rln is not extremely smaller than Rair, Rpkg must be extremely smaller than air resistance Rairout, but this is not practical. Consequently, if package 236 is made of material with low thermal conductivity, and the external wall of package 236 is exposed to non-homogeneous outdoor temperature due to heat from an external heat source, the package is susceptible to non-uniformity of the outdoor temperature. Therefore, device (LiNbO3 substrate 204) surface temperature is likely to be non-uniform (temperature gradient is generated).
Analyzing from the heat equivalent circuit shown in FIG. 32B indicates that ΔTs, which is a temperature difference between both ends of a device, cannot be reduced unless heat resistance of package 236 is extremely smaller than air resistance outside the package. Consequently, when heat-insulating material is used for package 236, problems would occur when the external temperature is non-uniform.
Therefore, even if soaking plate 232 is used, temperature of the entire LiNbO3 substrate 204 cannot be homogenized. Moreover, temperature gradient is found on the surface of SAW guide 211 and stress is applied non-uniformly due to temperature gradient. Therefore, generation of crystal strain caused by acousto-optical effects cannot be prevented strictly, and filter characteristics degrade. In addition, this further causes a detrimental effect when multi-channeling is attempted, and subsequently, the total length and breadth of LiNbO3 substrate 204 and SAW guide 211 increase.
In (2), it is possible to make the attenuation coefficient of SAW 1.3 dB/cm and suppress side lobes satisfactorily by carrying out annealing treatment for a specified time. However, this is an insufficient configuration from the viewpoint of achieving a uniform temperature distribution, and characteristic problems as described above remain unsolved.
The conventional example according to (3) intends to adjust the filter wavelength characteristics by changing the shape and arrangement of a strain providing section after manufacturing an element with the strain providing section for correcting local double refraction index of an optical waveguide. However, from the viewpoint of providing uniform temperature and achieving strain correction, it requires extra cost and impairs simplicity from the viewpoint of manufacture. Furthermore, it is extremely difficult to correct the difference in refraction index appropriately, in order to obtain completely satisfactory filter characteristics.
In the conventional examples described in (1) through (4), various techniques are described for improving characteristics as a device unit, but these methods do not consider any measures against heat when they are modularized. Particularly, these methods do not take into account generation of temperature gradient, and cannot solve the problem in the case of achieving multi-channeling in which a plurality of AOTF are positioned on one LiNbO3 substrate. As described in (4), it is well known that the filter characteristics of AOTF degrade when temperature distribution is present on the surface of the SAW guide. This is because when stress is applied non-uniformly due to temperature distributions, crystal strain due to acousto-optic effects becomes non-uniform and the filter characteristics degrade. Consequently, it becomes difficult to make the temperature in the SAW guide uniform. Furthermore, when multi-channeling is achieved, not only length but also width of the AOTF device increases, and it becomes still more difficult to achieve uniform temperature on the device surface.
The conventional examples described in (5) and (6) describe a soaking structure in a waveguide device. In these examples, a soaking plate made of a metal with excellent thermal conductivity is inserted between AWG waveguide device and temperature control device (heater, Peltier device). In the case of an AOTF device different from AWG, since the device area is large, there is a temperature gradient of the heater itself and temperature gradient due to heat resistance difference of air on the AOTF device surface. Therefore, the desired temperature uniformity cannot be achieved by this kind of soaking plate alone. That is, with such construction, a heat soaking plate is placed only on the Peltier device and though the temperature is made homogeneous, outside effects are not considered. In addition, if the heat soaking plate that provides good thermal conductivity is provided, consumption power increases.
In the optical add/drop modules that use an AWG device, described in (6), lengths of adjoining waveguides in the channel waveguides vary slightly and slab waveguide are formed on an optical substrate such as silicon, quartz, sapphire, etc. (see “FIG. 1” of Japanese Patent Application Laid-Open Publication No. 2000-249853). In this kind of waveguide type device, temperature distribution deviates the light path from the designed value due to temperature dependency of refraction index. In addition, in this waveguide type device, as the number of channels increases, the device area expands; and a construction that does not generate temperature distribution becomes indispensable.
Thus, the device area in AOTF devices is large. Therefore, problems such as temperature gradient of the heater and temperature gradient caused by difference in heat resistance on the device surface, etc. occur. Therefore, in AOTF devices, the use of the heat soaking plate alone cannot achieve the temperature uniformity in the device. On the other hand, in AWG devices, because the device area increases with the number of channels, temperature uniformity cannot be achieved in the device.